1. Field of the Invention
The disclosure relates generally to the field of sensors. More specifically, the present disclosure relates to a pressure sensor. Yet more specifically, the present disclosure concerns a pressure sensor within a wellbore and a method of using the pressure sensor.
2. Description of Related Art
Pressure gauges or sensors typically measure fluid pressure by communicating with the fluid whose pressure is being measured and transferring the pressure of the fluid being measured to a medium associated with the gauge. The medium generally reacts to the applied pressure, wherein a subsequent gauging of how the medium reacts can be correlated to provide a value for the measured pressure. An example of one such medium includes a fluid column, where the fluid density is known so that any corresponding change in elevation of the fluid column can easily be converted to a pressure reading. Other mediums include Bourdon tubes, springs, diaphragms, piezoelectric devices, piezorestrictive devices, and capacitive manometers.
Currently, some pressure sensors used in subterranean wellbores include a quartz resonator whose resonant frequency changes with pressure. However, a quartz resonator's resonant frequency also changes with temperature. Cutting the quartz crystal along different crystal axis can minimize its temperature sensitivity and maximize its pressure sensitivity. However, temperature compensation is still necessary and is generally accomplished using a second quartz resonator that is in thermal contact with the pressure-sensing resonator but which is not exposed to the high pressure of the downhole fluid. Sometimes a third, reference resonator is also used as a time base. For pressure sensing, the orientations include AT- or BT-cut. For temperature sensing, the orientations include AC- or BC-cut. For a time-base or frequency reference (which is not exposed to pressure) the orientation is generally SC-cut. Other pressure sensors used in downhole applications involve exposing an optical medium to fluid pressure, passing a light signal through the medium, and monitoring the behavior of the light passing through the medium. Examples of such optical mediums include a Fiber Bragg grating and a Fabry-Perot interferometer.
These pressure-sensing devices are subject to high ambient temperatures as well as to sudden but small variations in ambient temperature during sudden pressure changes, which are inherently adiabatic. As the temperature of the pressure sensor changes, the corresponding pressure reading changes too and a correction is necessary. However, when trying to monitor a pressure pulse as a function of time, as is often done, a complete temperature correction may not be possible because the local change in temperature of the pressure sensor cannot be corrected using the temperature sensor's reading because there is insufficient time for the temperature sensor to have reached thermal equilibrium with the pressure sensor.
Fiber Bragg Gratings (FBG) can be used as optical sensors for pressure and temperature and are typically made by inscribing a repeated pattern of refractive index variations into an optical fiber. These FBG production means are generally well known in the art such as by using the interference pattern of two intense UV laser beams. Changes in strain, stress, or temperature experienced by the fiber affect the light reflected from the FBG. The changes in light reflection are then correlated to changes in ambient conditions experienced by the FBG.
One example of a Fabry-Perot interferometer 54 is shown in FIG. 1. These devices typically comprise an optical fiber 56 having one end secured within a housing 64. The housing 64 also retains a diaphragm 60 shown opposite from the terminal end of the optical fiber 56. The housing 64 is configured to have a cavity 58 therein, although the optical fiber 56 extends a distance into the cavity 58, a gap 66 still exists between the terminal end of the optical fiber 56 and the diaphragm 60. The inner surface 62 of the diaphragm 60 is proximate to the fiber 56 and the outer surface 61 is subjected to pressure ambient to the interferometer 54. Light rays (R1 and R2) are emitted to the terminal end of the fiber 56 wherein at least a portion of these rays reach the inner surface 62 of the diaphragm 60. When light is passed through the fiber 56, a portion of the light reflects (R2) at the tip back into the fiber 56, and a portion (R1) passes through the tip to the diaphragm and reflects from the diaphragm back into the fiber 56. As R1 and R2 propagate back through the fiber 56, the gap in reflection distance between R1 and R2 results in measurable light interference between the two signals. Changes in pressure on the outer surface 61 of the diaphragm 60 affects the gap length 66 that in turn alters the measured light interference. The amount of interference can be correlated into the pressure exerted onto the outer surface 61 based on the change in gap width due to pressure changes.
The responsiveness of these devices can be affected by temperature. While temperature compensation means can be implemented for use with these devices, there exists a need for a pressure sensor whose readings are not in error and do not overshoot during pressure pulses and which have high resolution, high sensitivity, high dynamic range, and which can be operated at high downhole pressures and temperatures.